The use of laser pulses, in particular ultrashort laser pulses with a laser pulse duration in the range of between approximately 10−15 s and 5×10−10 s for the purposes of high-precision laser microstructuring operations is known.
G Mourou, in U.S. Pat. No. 5,656,186 (EP 0 754 103 B1, application filing date 8.4.94 ‘Method for controlling configuration of laser induced breakdown and ablation’), describes the basic suitability of individual ultrashort laser pulses for material processing.
In his dissertation relating to ‘Mikromaterialbearbeitung mit ultrakurzen Laserpulsen’, Cuvillier Verlag Göttingen, 1999, S Nolte discusses aspects of material processing using femtosecond laser pulses.
The treatment of vision defects in the human or animal eye by means of short laser pulse treatment is also previously known.
Thus in U.S. Ser. No. 005984916A (application filing date 20.4.93, ‘Ophthalmic surgical laser and method’) Shui T Lai describes the advantages of ultrashort pulses for refractive surgery on the eye.
In an article relating to ‘Application of ultrashort laser pulses for intrastromal refractive surgery’, Graefe's Arch Clin Exp Ophthalmol 238:33-39, 2000, H Lubatschowski et al describe the use of laser systems which produce ultrashort laser pulses of a duration of 100-200 femtoseconds, in the area of intrastromal refractive surgery.
Kurtz et al, ‘Optimal Laser Parameters for Intrastromal Corneal Surgery’, SPIE, Vol 3255, 56-66, January 1998, also use ultrashort laser pulses for tissue treatment.
For the treatment of vision defects, for example short-sightedness (myopia), various surgical interventions on the eye are employed to correct or at least reduce such defective vision. For example, in a first surgical treatment method (referred to as the ‘LASIK’ method), in a first step a cut is made with a special mechanical cutting device along a frontal plane through the cornea in order in that way to be able to lift off a flap of the cornea and fold it back.
Subsequently in a second step, usually starting from the cut surface of the body of the eye, a material removal operation is carried out in order to remove a lens-shaped piece of tissue. That material removal operation is either executed with the same mechanical cutting device as the first step in order to cut out the piece of tissue. In another operative procedure a second step involves material ablation by means of an excimer laser, which does not involve a cut but material vaporisation.
Then in a third step the corneal flap which had been folded away is folded back into place again and thus with its cut surface joins the surface produced by the lens-shaped cuffed-out configuration/the lens-shaped removal. The two wound surfaces are then joined together as they heal.
The optical properties of the cornea are changed by removal of the lens-shaped piece of tissue from the cornea. It is possible to achieve specifically targeted optical correction by virtue of a specific contour in respect of that lens-shaped piece of tissue.
In another procedure for treating vision defects the lens of the eye is removed and replaced by an artificial lens affording different optical properties. For that purpose it is also necessary in a first step to make an incision in the cornea of the eye in order to permit removal of the lens and fitment of the artificial lens.
Basically mechanical operating instruments are used for the first step in both of the above-mentioned methods as such instruments ensure an acceptable level of cutting performance and precision. The use of known laser cutting methods on the eye is limited as the cut surface quality is frequently not sufficiently high for the optical demands and the healing process is slowed down by virtue of thermal damage in the adjoining region of the cut surface or indeed complications can arise in regard to definitive healing of the operative incisions.
In general, that is to say also in accordance with the present invention, ultrashort laser pulses can be used for cutting, removing and structuring material (for example biological tissue) and for modifying material properties (for example for modifying the refractive index in glass).
The particular advantages of material processing with ultrashort laser pulses (fs-laser pulses) are found in particular in relation to the extremely precise cutting and/or ablation of materials, involving less damage both thermally and also mechanically, than in a series of other material processing methods. Due to focusing of the ultrashort laser pulses, energy is deposited at the focus in a very limited space by triggering of a microplasma and a cutting effect or material ablation is achieved by so-called photodisruption. It is possible to achieve removal rates in the sub-μm range with cutting widths of less than 500 nm. By virtue of a non-linear interaction mechanism involved in photodisruption, material removal is in that case substantially independent of the properties of the material. In particular, when using fs-laser pulses, it is also possible to deal with materials involving a high level of thermal conductivity (such as for example metals) and materials involving a low degree of laser light absorption (such as for example polymers or certain biological tissues).
As an alternative to ablation on the surface of a material to be treated, focusing into materials which are transparent for laser radiation (such as for example the cornea of an eye) also makes it possible to achieve a cutting effect in the interior of the transparent material (tissue).
One problem in material processing by means of laser is that material division is achieved by the laser energy in the irradiated region and in addition material changes are caused in adjoining regions, and such changes are generally undesirable. Those changes in material are decisively dependent in extent and magnitude on how high the energy of the laser beam and its temporal distribution is.
EP 1 284 839 A1 and U.S. Pat. No. 6,787,733 W2 describe a method of laser processing of materials, involving online control of the processing result in order to optimise material processing and to minimise unwanted side-effects. Admittedly, online control of the processing operation is possible with that method and in that case also any side-effects which occur can be observed, but it is not possible with that method to avoid surrounding regions of material being influenced by the laser radiation, besides the region which is to be treated thereby.
The interaction mechanism in tissue treatment by means of short intensive laser pulses is based on so-called photodisruption. Photodisruption occurs when light is focused to intensities of the order of magnitude of 1011 to 1012 W/cm2. At such high levels of intensity, almost any kind of material is abruptly ionised by virtue of multi-photon absorption (plasma generation). If that process takes place in transparent material (water, glass, cornea) it is referred to as ‘optical’ breakdown.
The explosive expansion of the laser-induced plasma causes the development of a strong pressure front and—if the process takes place in a fluid environment—a cavitation bubble. The pressure front as well as the cavitation bubbles can represent a considerable mechanical damage potential for surrounding material/tissue. That damage potential of an individual laser pulse scales however with the laser pulse energy.
A high level of intensity however is necessary to trigger optical breakdown. The shorter a laser pulse, the correspondingly less energy it transports, at the same level of intensity. Consequently, short laser pulses are a necessary prerequisite in order to implement photodisruption at a high level of precision and with slight mechanical side-effects.
The laser pulse duration, that is to say the time for which the laser radiation acts, is however also of crucial significance in regard to thermal damage to the irradiated material. If the radiation duration of the laser is so short that, during the irradiation procedure, no significant amount of thermal energy is lost due to heat diffusion from the absorption volume, that situation is referred to as ‘thermal confinement’. The maximum irradiation time (laser pulse duration) at which the conditions of ‘thermal confinement’ are maintained is also referred to as the thermal relaxation time τR and is given by the following relationship:
      τ    R    =            δ      2              4      ⁢      κ      In that relationship:
δ: denotes the optical depth of penetration into the irradiated material/tissue, in the case of disruption in transparent media δ corresponds to the expansion in the non-linear absorption zone; and
κ: denotes the thermal diffusion constant of the irradiated material/tissue.
If the time of action (laser pulse duration) is markedly shorter than τR, the zone of thermal influence which the laser pulse exerts on the irradiated material is determined primarily by the optical depth of penetration δ. In the case of a laser pulse duration which is markedly longer than τR, the zone of thermal influence acts beyond the optical depth of penetration by virtue of thermal diffusion.
When dealing with metals in respect of which an optical depth of penetration of only a few nanometers is involved and a comparatively high level of thermal diffusion prevails the use of pico- or even femtoseconds is therefore essential in order also to minimise the zone of thermal influence. Organic substances or biological tissue with a substantially lower level of thermal diffusion and with very much higher optical depths of penetration in contrast have thermal relaxation times in the region of nanoseconds or even microseconds.
However the reduction in the laser pulse duration is subject to technological limits at the present time and also does not prevent the surrounding regions being subjected to unwanted influences.
In principle to achieve material division it is necessary to introduce a certain amount of energy into the region to be divided, with the laser pulse. The level of that energy is dependent on the material. In the case of material division along a line or a surface, a plurality of laser pulses are used in mutually juxtaposed relationship, with the appropriate laser pulse energy, in order to achieve a division line or division surface. The disruptive effect of the individual laser pulses can give rise to unwanted mechanical changes in the material, depending on the respective nature of the material. Such changes include tearing effects in the tissue, which causes an irregular cut surface, or the formation of gas bubbles which can also detrimentally influence the geometry of the cut. In that case the extent of the unwanted changes in the material scales with the energy of the individual laser pulses.
By virtue of the versatile possible forms of use and the possibility of incisions which are controlled in an automated mode, lasers are already used for a series of incision procedures and removal procedures on the eye. A limitation in terms of possible forms of use is however imposed for example due to thermal tissue damage and the in part still unsatisfactory quality of the cut surface. There is therefore a need for improvement to the effect that the cleanness of the cut is increased, that is to say the degree of roughness of the cut surface is reduced and the evenness thereof is enhanced. There is also a need to provide a precise cutting method for refractive surgery, which allows the tissue surrounding the cut surface to be dealt with as gently and carefully as possible. The aim of the invention is to meet that need.
In a German patent application which has not yet been laid open, Lubatschowski describes a method of material and tissue division by means of irradiation of a region of the material with a succession of laser pulses, in which the laser pulse energy of an individual laser pulse of the succession of laser pulses is less than the laser pulse energy which is used to produce material division with an isolated laser pulse in the region irradiated with the individual laser pulse of the succession of laser pulses. That method is suitable for precision cutting of materials with laser pulses.
The merit of the inventor of this patent application is to recognise the advantageous use of that method for refractive surgery. In refractive surgery a deliberate cut is made in the corneal tissue of a human being or animal in order in that way to be able to correct a vision defect. Actual correction of the vision defect is effected in the case of the surgical treatment methods referred to hereinbefore by way of example, by cutting out in a lens form a part of the cornea or by removal and replacement of the lens. There is a need for treatment methods with which such a treatment can be carried out as gently and carefully as possible for the surrounding tissue and at the same time safely and effectively.